Abstract:
One might think that with over a
hundred Li-Polymer cells on the market,
one could find just the right cell for
every application, but this is far from the
case. Most cells are made in Asia by
companies that only sell in large
quantities. Dimensions are driven by the
size and shape that is appropriate for
3.6V hand held devices. For these devices thin and flat are paramount, while the linear
dimensions are driven by the size of the device and are usually too wide for a prosthesis. Figure
1 shows four cells that have proven usable in prosthetics. Left to right, the first cell is a mere
200 mAh, but is really small. The second, 250 mAh, is the smallest rectangle that will mount
crosswise at the end of a child prosthesis. Unfortunately, it has been discontinued by the
manufacturer. The third cell, 450 mAh, is slim in two dimensions so it fits well with one cell
each on side of a wrist. The 750 mAh cell on the right is good for a full day’s operation of an
adult transradial prosthesis.
Cells must be selected for their electrical characteristics as well as for size. For instance,
many of lithium ion cells have a high internal resistance limiting the current that can be drawn.
Recent changes in the chemistry of some of the lithium polymer cells have opened up new high
current applications. To move an elbow or shoulder with weight in the gripping device requires
substantial current even when extra cells are added to increase voltage. With battery operation
every component must be as efficient as possible and this includes the batteries.
